The problem I see with any life+ based duration, is it selectively rewards people who have a big hit that keeps coming back into print early in their careers, and then live a long time afterwards, and the converse of that is it punishes the author who doesn't have much success until late in life, or worse, gets his or her career cut short by a fatal illness. You've suggested a system that (sort of) fixes the later case, but it doesn't address the first half of the problem. Also, any life plus system is going to look like a better deal if the author has heirs he or she cares about, and less of a deal if they don't. If the whole goal under the Constitution is to provide an incentive, we have to look very carefully at how some people may or may not feel "incentivised".
To show you how your system might have worked if it had been in effect all along, lets take two Fantasy/SF/Horror authors:
First, H. P. Lovecraft. His first real hit of a story was 1926, with Call of Cthulhu. Just about everything that got reprinted when he first gained posthmous popularity was written after that. Then he died of Bright's disease, in 1937. Under the system of that time, most, if not all, of his work was still in copyright. But, it was still the great depression, and after that, there were the wartime paper shortages, so Life +10 would leave his work coming out of copyright just about when there starts being a chance of it getting printed. With your 40 year clause, some of his original copyrights would have lasted until about 1974, by which time he was starting to be reevaluated, and effectively expired just about the time his work finally caught on. Under the system actually in effect, most of his work was still under copyright until well after the first film adaptation (Dean Stockwell and Sandra Dee in the Dunwich Horror). He did not have any direct heirs, and probably would not have believed as he wrote his last works that there was any chance he was leaving a literary estate that might actually become worth more than the cost of a cup of coffee. His closest heirs were a pair of aging aunts, and by the time there were payments, they went to very distant relatives indeed.
Second, Michael Moorcock. He starts writing professionally at 15, and some of his biggest successes were written by the time he was 20. In his 70s now and still going strong, he'd enjoy life +10 on most of his work, and it's not inconceivable that Life +10 might apply even to his most recent books. I don't know if he even has direct heirs, but he has been married a couple of times and had some living relatives, so I suppose it's at least somewhat likely there are children, or perhaps nieces or nephews. Under the existing system, he would theoretically have a longer period of protection, but that may not matter in practical terms. The older US or British systems, current law, or your system are likely to leave him about the same, financially, but current law is, in theory, better for him. However, it's a mystery to many people why his work hasn't been optioned more by Hollywood, to the point of a completed film or six. Your system just might ding him financially, if there are people who are hoping to get film rights cheap after he dies - they could just wait 10 years and let copyright on such Characters as Elric of Melnebone expire completely. Rationally, a shorter term may matter not at all or a great deal to him, but not just for the money.